Norwegian Patent No. 176686 describes devices for installing sea cables or other elongated objects at the bottom of the sea from a cable ship. It is in particular described how the geographical coordinates for the touchdown point (TDP) may be determined for a cable during its installation using a touchdown point follower which comprises navigational devices.
U.S. Pat. No. 6,113,312 describes how a local remote operated vehicle (ROV) may be used during the installation of an elongate element, such as a submarine cable, on the seabed. A guide weight tracking gear is suspended from a ship and rides on the elongate element. The gear includes a garage for hosting a local ROV. The problem of obtaining the TDP is solved by using a local ROV for observing the TDP.
The present inventors have experienced that while installing a cable at large depths while being subjected to strong sea currents the cable can drift off out of control. The dynamic behaviour or the cable at the sea bottom represents a high risk of cable damage. A further risk is associated with a possible tangle of the cable with the ROV umbilical or other vertical elements, particularly if operated from the same vessel.
In European patent application publication number EP 0431279 A1 there is proposed a cable guiding means with a weight large enough to secure a substantially vertical position relatively to a laying vessel, when the actual sea currents and depth conditions are taken into consideration. The guide weight (GW) is attached to the installation vessel by wires. The wire may support an umbilical for carrying control signals and power for electronic instrumentation included in the guiding means. The electronic instrumentation may include observation, navigation and communication functions. Thrusters or a rudder may manoeuvre the GW subsea.
The advantages of using a guide weight compared to an ROV are strength and simplicity. Vertical movements are controlled by a winch on the vessel, so the GW therefore has high lifting capacity as compared to an ordinary ROV. The downward vertical force may also be made large compared to most ROVs which typically are buoyant. The GW has no need for buoyancy and there is no need for thrusters to control vertical movements. The major parts of the horizontal movements are controlled by the vessel.
A GW umbilical is strong and drag displacements are limited due to the weight of the wire and the GW itself. There is no requirement for umbilical buoyancy as for an ROV umbilical. The GW may therefore also be used for lifts to reduce the use of a crane/winch or save some time by parallel installation/retrievals.
The GW defines a “fixed” point following the vessel movements close to the sea bottom. However, the cable to be installed must be controlled all the way from top to bottom to eliminate the risk of tangling.
In EP 0431279 A1 it is proposed to guide the cable through rings attached between the two vertical wires. The limitation of such a solution is that the cable is trapped to the installation tool all the way from top to bottom. This concept is most suitable when the cable is installed from one shore to the other. Discontinuities such as a sensor station on a seismic cable may be expected to represent a problem when passing the rings.
Another option is to increase the tension in the cable so that it becomes almost parallel with the GW wire, as presented in U.S. Pat. No. 6,113,312. The excess loop of cable due to sea currents will then be strongly reduced. A limitation of a high tension in the cable is that the touchdown point (TDP) will typically be far behind the GW, making it more complicated to observe the TDP, and the GW will be pulled backwards with respect to the vessel. The dynamic movements of the vessel due to weather (primarily waves) will make the position of the GW and TDP move quite a lot in rough water.
Although the prior art presents significant progress in the area of installing cables on the seabed, there are still limitations, in particular related to installation in deep waters with strong sea currents and rough seas.
Hence, a device and method that increase the reliability and efficiency of the deployment of a subsea cable would be advantageous, and in particular devices and methods for more efficient and/or reliable control of the cable during deployment at the sea bottom would be advantageous.